Recognition of nucleoplasmin by its nuclear transport receptor importin α/β: insights into a complete import complex.

Nuclear import of the pentameric histone chaperone nucleoplasmin (NP) is mediated by importin α, which recognizes its nuclear localization sequence (NLS), and importin β, which interacts with α and is in charge of the translocation of the NP/α/β complex through the nuclear pore. Herein, we characterize the assembly of a functional transport complex formed by full-length NP with importin α/β. Isothermal titration calorimetry (ITC) was used to analyze the thermodynamics of the interactions of importin α with β, α with NP, and the α/β heterodimer with NP. Our data show that binding of both importin α and α/β to NP is governed by a favorable enthalpic contribution and that NP can accommodate up to five importin molecules per NP pentamer. Phosphomimicking mutations of NP, which render the protein active in histone chaperoning, do not modulate the interaction with importin. Using small-angle X-ray scattering, we model the α/β heterodimer, NP/α, and NP/α/β solution structures, which reveal a glimpse of a complete nuclear import complex with an oligomeric cargo protein. The set of alternative models, equally well fitting the scattering data, yields asymmetric elongated particles that might represent consecutive geometries the complex can adopt when stepping through the nuclear pore.